1. Field of the Invention
This invention relates to a laser scanning type observation apparatus, such as laser scanning microscope and laser scanning endoscope, in which a pulsed laser is irradiated to a test object and then detection signals acquired by receiving light from the test object are sampled.
2. Description of Related Art
In general, as shown in FIG. 1 for example, a laser scanning microscope is configured to include: a light source 51 generating laser oscillation; a half mirror 52 splitting a laser beam from the light source 51 and splitting light from a test object 59; a galvanometer mirror 53 performing laser beam scanning in the X-direction and in the Y-direction; an objective lens 54 irradiating the laser beam to the test object 59; a pinhole 55 eliminating scattered light or the like from the light traveling from the test object 59 to extract only light traveling from a focal position; a light-receiving device 56 receiving the light that has passed through the pinhole 55; and an image-processing unit 57 to which analog electrical signals generated by photoelectric conversion by the light-receiving device 56 are supplied. The image-processing unit 57 converts the supplied analog signals into digital signals with a sampling clock optionally determined by an independent oscillator or the like, and then the converted digital signals are stored in a memory that is not shown in the drawings. And then, an image which is made to synchronize with a scan with the laser beam is formed with the digital signals stored in the memory, and the formed image is displayed on a monitor display 58.
Continuous wave lasers are commonly used for laser scanning microscopes. However, laser beams with various wavelengths have been used in recent years because of diversification of test objects for research, and laser beams are often generated also by causing pulse oscillation with a mode-locking method or the like. And, for example, mode-locking ultra-short pulsed lasers have been used in order to detect multi-photon fluorescence from a test object.
In a scanning microscope with pulsed light, pulsed light is irradiated to a test object, light from the test object is received by a light-detecting unit, detection signals outputted by the light-detecting unit are sampled, and an image is formed by using the intensities of the sampled detection signals as pixel values. In this case, the detection signals acquired by receiving the light from the test object become signals the light intensities of which attenuate as time passes by.
Accordingly, if the detection signals from the light-detecting unit are sampled just as the intensities of the detection signals have the maximum values respectively, it is possible to capture images with the highest sensitivity of detection.
As shown in FIG. 2 for example, the following Japanese Granted Patent No. 4667571 discloses a laser scanning microscope which includes: a laser device 61 playing a role as a pulsed laser oscillation means that generates pulsed laser oscillation; a photoelectric conversion unit 62 playing a role as a light-detecting unit that receives light from a test object 70 to output electrical signals; an A/D converter 63 playing a role as a sampling means that samples the electrical signals from the light detecting unit 62; a memory 64 storing data that are sampled by the sampling means 63; and a laser-oscillation-synchronizing-signal generating circuit 66 playing a role as a synchronous signal-generating means that outputs a synchronous signal made to synchronize with oscillation of pulsed laser in response to laser oscillation signals detected by a laser-oscillation detecting unit 65 detecting the pulsed laser oscillation. And, the laser scanning microscope disclosed in Japanese Granted Patent No. 4667571 further includes: a delay circuit unit 67 delaying the synchronous signal for a set amount of time to output trigger signals; and an external input circuit 68 for providing an amount of delay time Δt with which the synchronous signal is delayed by the delay circuit unit 67, wherein the sampling means 63 samples the signals while the trigger signals from the delay circuit unit 67 are being used as a sampling clock. And, Japanese Granted Patent No. 4667571 suggests a method of performing sampling that is made to synchronize with an oscillation mode of laser in order to efficiently acquire detection signals in detection of multi-photon fluorescence. Besides, in FIG. 2, the numeral reference 69 denotes an image-displaying unit that displays an image formed by detection signals stored in the memory 64.
In the laser scanning microscope disclosed in Japanese Granted Patent No. 4667571, detection signals after photoelectric conversion are chronologically compared with trigger signals, and then a value of delay time which is inputted through the external input circuit 68 is changed so that peaks of the detection signals correspond with timing of the trigger signals outputted or so that images have the brightest brightness. As a result, it is possible to adjust timing at which the detection signals are sampled just as their intensities have the maximum values, in the laser scanning microscope disclosed in Japanese Granted Patent No. 4667571.